Green’s Dictionary of Slang

back-hand v.2

[back-hander n. (3)]

to give a bribe.

1961
197019801990
1995
[Ire]T. Murphy Whistle in the Dark Act I: Too much back-handin’, too much palm-oil.
[UK]J. Cameron Vinnie Got Blown Away 72: Maybe backhand a screw bring some in.